Bond of union (7)
I believe the answer is:
liaison
This clue is a double definition.
'bond' is the first definition.
(I have seen 'Bond of union' mean 'liaison' so perhaps 'bond' could also mean 'liaison')
'union' is the second definition.
(I've seen this before)
'of' acts as a link.
(Other definitions for liaison that I've seen before include "Connection - relationship" , "Amour; communication" , "Secret love-affair" , "Co-operation; love affair" , "Naughty affair?" .)